1. Field of the Invention
The present invention relates to direct current (DC) power control, and in particular, to a DC power control circuit for converting an input DC voltage into a predetermined output DC voltage.
2. Description of the Prior Art
Conventionally, there has been utilized a forward-bias-type DC power circuit which includes a transformer for converting an input DC voltage, variable in a wide voltage range, into a predetermined output DC voltage. Among the DC power circuits of this type, there is a power circuit having a transistor serially connected to a primary winding of a transformer. Conductivity of the transistor is controlled, for example, in the manner of a pulse-width modulation (PWM) in response to an output voltage of the circuit, thereby producing a predetermined DC output voltage across the secondary winding regardless of the input DC voltage. Both step-up and step-down functions with respect to the input voltage can be advantageously achieved by a single circuitry constructed in a relatively simple configuration. On the other hand, however, the DC power circuit has such problems that leaking magnetic flux of the transformer causes noise within the electronic circuit disposed in the vicinity thereof, and additionally only a low power conversion efficiency between the input and output can be obtained.
In order to solve the problems, there has been widely utilized a DC power control circuit comprising a choke coil in place of the transformer. This type of circuit includes a transistor serially connected to the choke coil, and the step-up and step-down of the input voltage are carried out by driving the transistor using a pulse-width modalation technique in response to an output voltage. The fly-back characteristic of the choke coil is utilized in the circuit, which advantageously develops a higher power conversion efficiency and a lower leakage flux. However, such a single power control circuit has to be dedicated to the step-up or step-down function. That is, this circuit cannot be adopted in an application in which an input voltage may change over and under with respect to an expected value of the output voltage from the power circuit.